Along with microfabrication of a semiconductor device, processing accuracy required for an etching process of a sample has been increasingly strict. In order to perform a high accuracy process on a micropattern of a wafer surface with a plasma processing apparatus, it is important to control the temperature of the wafer surface during an etching process. However, due to the demand for a larger area of a wafer and the improvement of an etching rate, high frequency electric power applied to the plasma processing apparatus tends to be increased, and large electric power in kilowatt-order has been beginning to be applied, in particular, for etching of an insulating film. The applying of large electric power increases ion impact energy to the wafer surface, which involves a problem of an excessive rise of the temperature of the wafer during an etching process. Further, due to the demand for further improvement of shape accuracy, a means capable of controlling the temperature of the wafer at high speed and with accuracy during the process has been demanded.
In order to control the surface temperature of the wafer in a plasma processing apparatus, it is necessary to control the surface temperature of an electrostatic absorption electrode (hereinbelow, referred to as an electrode) which comes in contact with the back surface of the wafer through a heat transfer medium. For a conventional electrode, channels for a refrigerant are formed inside the electrode, and a liquid refrigerant is allowed to flow inside the channels so as to control the temperature of an electrode surface. The liquid refrigerant is supplied to refrigerant channels inside the electrode after being adjusted to a target temperature by a cooling apparatus or a heating apparatus inside a refrigerant supply apparatus. Such a refrigerant supply apparatus has a structure in which the liquid refrigerant is once stored in a tank and is fed after adjusting its temperature, and is effective in keeping the surface temperature of the wafer constant because the liquid refrigerant itself exhibits a large heat capacity. However, the liquid refrigerant is poor in temperature response, low in heat exchange efficiency, and the temperature thereof is difficult to control at high speed. Therefore, the refrigerant supply apparatus becomes larger in size along with a recent high-heat-input, and it has been difficult to optimally control the temperature of the wafer surface in accordance with the progression of etching.
For the problems described above, there has been proposed a direct-expansion-system refrigerant supply apparatus (hereinbelow, referred to as a direct-expansion-system refrigeration cycle) in Japanese Patent Application Laid-Open No. 2005-89864 in which a compressor for allowing the pressure of a refrigerant to be higher with a refrigerant circulation system, a condenser for condensing the high-pressured refrigerant, and an expansion valve for expanding the refrigerant are installed in an electrode, and the refrigerant is evaporated inside refrigerant channels of the electrode for cooling.
In the direct-expansion-system refrigeration cycle, since latent heat generated by refrigerant evaporation is utilized, the cooling efficiency is high, and further it is possible to control the evaporation temperature of the refrigerant by pressure at high speed. For the reason as described above, by employing the direct-expansion-system refrigeration cycle as a refrigerant supply apparatus for an electrode, the temperature of the semiconductor wafer at the time of a high heat input etching process can be controlled with high efficiency and at high speed.
The direct-expansion-system refrigeration cycle performs cooling by using latent heat generated when the refrigerant evaporates from a liquid state to a gaseous state, and the evaporation temperature of the refrigerant can be controlled by pressure. In the refrigerant channels of the electrode, if the refrigerant pressure is constant, the evaporation temperature is constant. However, the refrigerant flows while being evaporated by absorbing heat in the channels, and thus a heat transfer coefficient is changed along with the phase change. That is, even when the refrigerant pressure is kept constant in the refrigerant channels in consideration of uniformity of the in-plane temperature of the electrode, the heat transfer coefficient becomes nonuniform in the refrigerant channels, and thus it is difficult to uniformly control the surface temperature of the electrode, and further the in-plane temperature of the wafer. For that reason, when the direct-expansion-system refrigeration cycle is employed as a cooling mechanism for an electrode, the uniform control of in-plane temperature distribution is a technical challenge.
For the above described problems, Japanese Patent Application Laid-Open No. 2005-89864 proposes a method in which by using a heat diffusion plate for an electrode surface on which the wafer is placed, the nonuniform heat transfer of the refrigerant is corrected so that the in-plane temperature of the wafer is made uniform. Thereby, even if the direct-expansion-system refrigeration cycle is employed as a cooling mechanism for an electrode, the in-plane temperature of the wafer can be uniformly controlled with high cooling efficiency. However, in the case where the temperature of the wafer is controlled at high speed hereafter, it is necessary to reduce the heat capacity of the electrode. Even if the evaporation temperature of the refrigerant can be varied at high speed, the large heat capacity results in lowering of the speed of the temperature control for the wafer.